This disclosure relates generally to the field of mapping induced fractures in subsurface formations, more specifically, the disclosure relates to method for identifying volume of fractures induced, for example, by hydraulic fracturing using passive seismic signals detected above the formation in which the fractures are induced.
Passive seismic emission tomography is a technique that is used for, among other purposes, determining the hypocenter (i.e., place and time of origin) of microearthquakes resulting from formation fracturing that occurs in subsurface rock formations. Such microearthquakes may be naturally occurring or may be induced, for example, by pumping fluid into formations at sufficient pressure to cause failure, i.e., fracturing of the formation. In the latter case, it is useful to be able to determine progression of the fluid front as the fluid is pumped into the formations. One technique for performing such fluid front determination during fracture pumping is described in U.S. Pat. No. 7,663,970 issued to Duncan et al. incorporated herein by reference in its entirety. The technique described in the Duncan et al. '970 patent may be used to determine hypocenters of microseismic events (or microearthquakes) caused by failure of the subsurface rock formations as hydraulic fracturing fluid is pumped into the formations.
It is known in the art to generate maps of fracture networks induced by hydraulic fracturing from detected passive seismic signals. One such technique is described in U.S. Patent Application No. 2011/0110191 filed by Williams-Stroud, incorporated herein by reference in its entirety. Although effective at mapping most of the induced fractures in a discrete fracture network (DFN) using passive seismic signals, a large number of small fractures may not be determined using such technique or any other technique because the signals generated by the fracture events may not be detected. Thus, estimates of the total fracture volume may not correspond well with the volume of fluid pumped into subsurface formations.
What is needed is a technique that can be used to more accurately determine the total volume of fractures induced by hydraulic fracturing operations.